1. Field of the Invention
The present invention relates to a worm speed reducer and a method for manufacturing a worm wheel included in the worm speed reducer.
2. Description of Related Art
In an electric power steering system, rotation output from an electric motor is transmitted to a worm shaft in a worm speed reducer. The rotation output from the electric motor is decelerated via a worm wheel that meshes with the worm shaft, and the resultant rotation is transmitted to a turning mechanism. Thus, a steering operation undergoes torque assistance. The meshing between the worm shaft and the worm wheel needs backlash. However, during traveling, tooth hitting sound (rattle sound) may occur due to the backlash.
Consequently, an electric power steering system has been proposed in which the worm shaft is supported such that a first end thereof pivots around a second end thereof, with the second end elastically biased by a bias member toward the worm wheel side, thus eliminating the backlash. However, only when the worm wheel rotates in a first rotating direction, a meshing reaction force (driving reaction force) that the worm shaft receives from the worm wheel at a meshing portion of the worm shaft results in a moment that biases the meshing portion of the worm shaft toward the worm wheel side (that is, in the direction in which the first end is biased by the bias member) around the second end.
Therefore, meshing frictional resistance resulting from rotation of the worm wheel in the first rotating direction is larger than meshing frictional resistance resulting from rotation of the worm wheel in a second rotating direction that is an opposite direction of the first rotating direction. Thus, frictional resistance torque in the former case is higher than frictional resistance torque in the latter case. As a result, steering feeling may vary according to a steering direction.
On the other hand, two techniques for an electric power steering system have been proposed; one of the techniques involves varying a pressure angle between tooth flanks forming a pair of tooth flanks of the worm shaft, and the other involves varying the pressure angle between opposite tooth flanks of the worm wheel in the rotating direction (see, for example, paragraph [0041] and FIG. 5 of Japanese Patent Application Publication No. 2006-103395 (JP 2006-103395 A)). JP 2006-103395 A is expected to be effective for suppressing the difference in frictional resistance torque due to a difference in rotating direction.
However, for the worm shaft and the worm wheel, a cutting tool with a special shape is needed to vary the pressure angle between the tooth flanks. That is, a plurality of cutting tools is needed in which the cutting tools have different shapes corresponding to different specifications for the worm shaft or the worm wheel. This leads to high overall manufacturing costs.